Plasma spray is perhaps the most flexible of all of the thermal spray processes as it can develop sufficient energy to melt any material. Plasma spray guns use powder as the coating feedstock, and the number of coating materials that can be used in the plasma spray process is almost unlimited. In a plasma spray gun, a high frequency arc is ignited between an anode (nozzle) and a cathode (electrode). Process gases, generally mixtures of argon, nitrogen, hydrogen and helium, flowing between the anode and the cathode is ionized to become a plume of hot plasma gas, reaching temperatures of 6,600° C. to 16,600° C. (12,000° F. to 30,000° F.). When the coating feedstock material is injected into the gas plume, it is melted and propelled towards a target substrate.
The plasma plumes produced by atmospheric thermal spray guns that are laminar are sensitive to the surrounding environment. These plumes can be easily disrupted by air currents. As the thermal spray gun is moved or traversed over a substrate, air currents impinge upon the plume. Compounding this is the need to have a forced flow of air through a spray booth during operation of the plasma gun. Disturbing the plasma plume can cause several undesirable effects.
In a first undesirable effect, the plume can be shifted in a direction relative to powder injection resulting in poor energy transfer to the powder particles and subsequent poor coating results.
In another undesirable effect, the laminar tube of the plume can collapse and result in increased interaction with the environment. Laminar plumes are considered very effective for spraying as the energy is contained within the laminar tube and thus transfers more heat energy to the powder particles.
In another undesirable effect of extreme cases, for example with the external arc plumes, the arc position can be affected resulting in damage to the cathode, anode, and/or the surrounding components from misplaced arc energy.
Accordingly, what is needed is an apparatus that protects a laminar plasma plume from being disturbed from the ambient air currents and/or which overcome one or more of the above-noted deficiencies.